Structural , Spectroscopic and Biological Aspects of O , N-Donor Schiff Base Ligand and its Cr ( III ) , Co ( II ) , Ni ( II ) and Cu ( II ) Complexes Synthesized through Green Chemical Approach

The monofunctional bidentate Schiff base ligand (o-vanillin) p-chloroaniline and its four new complexes of chromium(III), cobalt(II), nickel(II) and copper(II) have been synthesized by classical thermal and microwave-irradiated techniques. All the new derivatives have been characterized by elemental analysis, molecular weight determinations, molar conductance measurements, UV-Vis, IR and NMR spectral studies. The IR spectral data suggest the involvement of phenolic oxygen after deprotonation and azomethine nitrogen in coordination to the central metal ion. The growth inhibiting potential of the ligands and complexes has been assessed against a variety of fungal and bacterial strains.


Introduction
The growing interest in Schiff base compounds is basically because of their raised biological activity and also they make possible development of novel materials with unique properties 1 .Many applications of microwaves, as an efficient heating source for organic reactions, have been reported in the literature 2 .Solvent free reaction have played strategic role in methodologies of organic as well as inorganic synthesis 3,4 .Microwave heating has proved to be very useful tool to carry out certain organic transformations which not only excludes the use of hazardous non-eco friendly solvents but also enhances the reaction rates greatly.The main advantage of microwave assisted organic synthesis is the shorter reaction time using only small amount of energy.The concept of green chemistry is enshrined the prevention of waste, the use of safe, environmentally being solvent where possible and use on renewable feed stocks 5 .Schiff base have often been used as chelating ligands in the field of coordination chemistry and their metal complexes are of great interest for many years 6 .It is known that the existence of metal ions bonded to biologically active compounds may enhance their activities 7,8 .The transition metal Schiff base complexes are applicable in areas such as bioinorganic, electrochemistry, environmental chemistry, metallic deactivators and separation technique 9 .

Experimental
All the reagents were dried and distilled before use o-vanillin and p-chloroaniline were purchased from Hi-media.Cupric acetate, cobalt acetate and nickel chloride were purchased from E.Merck.Chromium chloride was purchased from Thomas Baker.

Preparation of ligand
In microwave assisted synthesis the ligand was prepared by the condensation of respective aldehyde and amine in 1:1 molar ratio using a beaker through a conventional microwave oven by taking few mL solvent (ethanol), respectively.The reaction was completed in a short period of 2 minutes.The solution when cooled gave orange needle shaped crystals.These are washed in ethanol and dried over anhydrous CaCl 2 .
For comparison purpose, the above ligand was also synthesized by the thermal method, where instead of few drops of alcohol the starting material of the ligand was dissolved in 30ml of alcohol and the content was refluxed for 1.5 hours.The solution was cooled to room temperature to obtain the orange needle shaped crystals.

Preparation of the complexes
The complexes were also prepared by two different types.In microwave assisted synthesis, the complexes were prepared by irradiating the reaction mixture of metal salts (ethanol and double distilled water) and respective ligand in 1:2 molar ratios in benzene.The reactions were completed in 5 -8 minutes.On cooling, the coloured complex precipitated out, which was filtered by suction, washed several times with ethanol and finally ether and dried over anhydrous CaCl 2 .
These complexes were also prepared by the thermal method where instead of 5 -8 minutes reaction was completed in 3 to 4.5 hours and yield of the products was also less than that obtained by the microwave assisted synthesis.The purity of the compounds was checked by TLC.A comparison between thermal and microwave method is given in Table 1.

Physical measurements and analytical data
Microanalyses of carbon, hydrogen and nitrogen of the complexes were carried on a Heraeus Carlo Erba 1108 elemental analyzer.Metals content were analyzed gravimetrically.IR spectra were recorded on a Perkin-Elmer infrared spectrophotometer in the range 4000-400 cm -1 .
Electronic spectra of the complexes were recorded on a Helios alpha. 1 H NMR spectrum of the ligand was recorded on a Bruker Avance 400 MHz.Molar conductances were measured at room temperature in DMSO using a dip type cell electrode.The molecular weights were determined by the Rast Camphor method.

Anti-bacterial studies
The antibacterial activity was evaluated by the paper disc method.The nutrient agar medium (peptone, beef extract, NaCl and agar -agar) and 5 mm diameter paper discs of Whatman No. 1 were used.The compounds were dissolved in DMSO in 500 and 1000 ppm concentration.The filter paper discs were soaked in different solutions of the compounds, dried and then placed in the petriplates previously seeded with the test organisms E. coli, S. aureus.The petriplates were stored in an incubator at 28 ± 2 0 C for 24 hours 10 .

Anti-fungal studies
Antifungal activities of the ligand and its complexes have been evaluated against two pathogenic fungi, Aspergillus nigre and Fusarium oxysporum by the agar plate technique.The potato dextrose agar (PDA) medium was prepared in the lab to maintain the fungal growth.Solutions of the test compounds in DMSO at 100 and 200 ppm concentrations were prepared and then were mixed with the medium.The medium was poured into petriplates and the spores of fungi were placed on the medium.These petriplates were wrapped in the polythene bags containing a few drops of the alcohol and were placed in an incubator at 25 ± 2 o C. The activity was determined after 96 hours of incubation at room temperature 25 o C 11 .

Figure 1
The metal chelates in the present investigations were prepared according to the equation:

Physical properties
The resulting complexes are air stable, coloured solids.The observed molar conductance (7.8 to 10.4 ohm -1 cm 2 mol -1 ) of all the complexes in 10 -3 M dimethyl sulfoxide solutions suggests the non-electrolytic 21 nature of these complexes.The analytical results (thermal& microwave method) of ligand and its complexes are presented in

Infrared spectra
The important IR spectral bands of the ligand as well as complexes are presented in Table 4.The IR spectra of the metal complexes show significant changes compared to the ligand.In the IR spectrum of the Schiff base ligand, a sharp band observed at 1616 cm -1 is assigned to the √(C=N) made of azomethine group 22 .This shifts to lower frequency by 4-6 cm -1 in all the complexes, suggesting the coordination of the azomethine nitrogen to the metal centers.This is further substained by the presence of a new band at 491-476 cm -1 assignable to √ (M-N) 23.The characteristic phenolic √ (O-H) mode due to the presence of a hydroxyl group at ortho position in the ligand was observed at 3414 cm -1.A band at 1298 cm -1 due to √(C-O) phenolic was observed in the ligand.The disappearance of the phenolic √ (O-H) band in the complexes under study suggests the co-ordination by the phenolic oxygen after deprotonation to the metal ion.This is further supported by the shifting of the √(C-O) phenolic to higher wave numbers in all the metal complexes.The complexes also showed medium intensity bands in the region 580-585 cm -1 , which can be attributed to the respectively √(M-O) vibrations.The presence of lattice water molecules in the complexes is revealed by the appearance of a broad band at 3500 cm -1 due to √ (O-H) 24 .
Table 4. IR spectral data of ligand and its metal complexes

H NMR spectra
The 1 H NMR spectrum of the ligand was recorded in CDCl 3 -d 6 .The proton signals at 7.246, 3.941, 6.902-7.406and 8.622 δ ppm are due to the solvent, OCH 3 group, aromatic protons and azomethine (-CH=N-) proton, respectively.The signal at13.391δ ppm is due to phenolic (-OH) proton of vanillin 25 .Thus, on the basis of the above studies the expected structures of the complexes may be represented as shown in Figure 2.

Anti-microbial studies
The ligand and its metal complexes were evaluated for their antimicrobial activity against two bacteria Escherichia coli, Staphylococcus aureus and two fungi, Aspergillus niger and Fusarium oxysporum.The results are summarized in Table 5&6.The results were compared with those of the standard drug streptomycin for bacteria and flucanazole for fungi.

Conclusion
Microwave irradiation is an efficient and environmentally benign method to accomplish various inorganic synthesises to afford products in higher yields in shorter reaction periods.The metal complexes synthesized in 1:1 molar ratios with respective ligand were found to be tetracoordinated and hexacoordinated.The ligand behaves as a bifunctional tetradentate with the metal atom.Antimicrobial activity showed that the complexes are more active than the parent ligand.

Figure 2 .
Structures of the complexes

Table 1 .
Comparison between conventional and microwave methods of synthesis

Table 3 .
Physical and analytical data of the ligand & its complexes synthesized by microwave method

Table 5 .
Antibacterial activity of the Schiff base ligand and their metal complexes, showing of diameter of inhibition zone

Table 6 .
Antifungal activity of the Schiff base ligand and their metal complexes, showing of diameter of inhibition zone